The present invention relates to processes for the production and deposition of monomolecular layers on a substrate and more particularly relates to a process and an apparatus for producing alternate monomolecular layers, i.e. sequences of layers of two different materials.
Monomolecular layers, particularly of organic compounds, have numerous applications, especially in the electronics field, where they make it possible to produce metal-insulant-metal structures in which the insulant layer has a perfectly controlled and defined thickness.
The best known process for producing monomolecular layers is the so-called Langmuir-Blodgett process, which consists of spreading a solution of the molecule to be deposited over the surface of a liquid, generally water. This process is illustrated by FIGS. 1a to 1e. The solution is spread over the surface (FIG. 1a), followed by the evaporation of the solvent to form the monomolecular layer, which is then compressed at a given surface pressure by the displacement of a floating barrier B (FIG. 1b). Deposition takes place by immersing and then raising again the substrate S in the water of the vessel through said monomolecular layer (FIGS. 1c, 1d and 1e). Generally, the molecules of the layer to be deposited are amphiphilic molecules, i.e. they have both a hydrophilic part and a hydrophobic part. Following the evaporation of the solvent, the molecules are oriented in such a way that the hydrophilic part is located against the surface of the water, whilst the hydrophobic part tends to move away therefrom (FIG. 1b). Usually, the hydrophobic part is constituted by an inert aliphatic chain and the hydrophilic part contains the active part of the molecules.
The standard procedure for compressing the monomolecular layer consists of displacing a floating barrier on the surface of the liquid as deposition is taking place on the substrate, in order to maintain the surface pressure at the desired value. At the end of the operation, the mobile barrier is returned to its initial position, so that the operation can be recommenced by introducing a further solvent quantity onto the surface of the water. Although this process makes it possible to easily produce monomolecular layers, it is apparent that it cannot be used for ensuring the production and deposition in a continuous manner of monomolecular layers on solids having a considerable length, such as plastic or metal tapes or strips. Thus, it is necessary to interrupt the operation every so often in order to resupply the bath with the solution of the material to be deposited.
French Pat. No. 2,341,199, belonging to the present Applicant, describes an improvement to this process resulting from the use of rotary rollers, which are partly immersed in the water of of a compartmentalized vessel making it possible to transfer the monomolecular layer from one compartment to the other under pressure. It is thus possible to carry out continuous deposition on plastic or metal strips or bands. The film which can be obtained by one or other of these two processes can be transferred onto a solid substrate by merely passing the latter through the surface of the water. As a result of successive immersions and emersions, other monomolecular layers, identical to the first, can be deposited the hydrophilic part of one layer being in contact with the hydrophilic part of the following layer and the same applies with respect to the hydrophobic parts. The thus obtained deposit is called the Langmuir-Blodgett or L.B. film.
However, this process still suffers from a disadvantage. Although it makes it possible to carry out continuous deposition, it only makes it possible to produce Langmuir-Blodgett layers which are identical to one another. It is also possible to envisage the successive use of two independent vessels with an overhead transfer of the sample, but this only makes it possible to achieved paired sequences and not alternate sequences, because the contacting hydrophilic parts necessarily belong to two identical molecules.